I modded the game a bit by creating some characters and modifing others, saved the esp file, loaded it with fmm and tryied to load both last and first,
but the changes won't show in game and the characters i modded are still like in vanilla.

The characters could be saved in your save game, which would override changes to the characters in your game.
Or, your using a newer version of the GECK with an older version of the game.
Or, there are errors on your mod, like trying to mod a mod, that cannot be done. You can only mod masters.

I've been trying to see the changes in my old save, nothing. I started a new game, nothing. I'm using the least version of patches and geck witch are 1.4, and I modded the master file

Are you using some form of Archive Invalidation?
You need to if your replacing vanilla objects or textures.

Indeed I am, and thanks for the replies

I'm using other mods as well, maybe it is a conflict? I'm using Ling's mod to have more hair options, so I loaded its master file to mod some characters to make them more beautiful.
Isn't this what the mod is supposed to do?